I. Field
The present invention relates generally to circuits, and more specifically to techniques for performing direct conversion using variable amplitude local oscillator (LO) signals.
II. Background
In a wireless communication system, data is normally processed (e.g., coded and modulated) digitally and then frequency upconverted from baseband to radio frequency (RF) to obtain an RF modulated signal more suitable for transmission over a wireless link. The frequency upconversion may be performed using various transmitter architectures. For a super heterodyne transmitter architecture, the frequency upconversion is performed by at least two stages—typically from baseband to intermediate frequency (IF) by a first stage, and then from IF to RF by a second stage. For a direct conversion transmitter architecture, the frequency upconversion is performed by a single stage—from baseband directly to RF. Each stage requires an LO signal to perform the upconversion from an input frequency (which is either baseband or IF) to an output frequency (which is either IF or RF).
An important design consideration for a transmitter is the amount of LO leakage in the RF modulated signal. An ideal frequency upconversion stage would receive an input signal and an LO signal and generate an output signal that is simply the input signal translated in frequency by the LO signal's frequency. However, because of imperfections in circuit components and/or due to circuit layout, some of the LO signal inevitably leaks into the output signal. The leaked LO signal acts as noise in the output signal and may further cause other degradation.
LO leakage is more problematic for a direct conversion transmitter because the frequency of the LO signal is set to the desired RF output frequency. For this transmitter, the LO signal can couple to the output signal resistively (e.g., via a silicon substrate) and/or magnetically (e.g., via inductors used for the LO circuit and the transmit signal path). In contrast, for a super heterodyne transmitter, the LO signals for the stages are not at the desired RF frequency, and LO coupling and gain distribution in the transmit signal path are normally such that LO leakage is not as severe.
LO leakage is also more problematic for a transmitter that is required to provide a wide range of adjustment in output power. One such application that requires this wide power adjustment is a Code Division Multiple Access (CDMA) communication system. On the reverse link in the CDMA system, the signal from each terminal is spectrally spread over the entire (e.g., 1.2288 MHz) system bandwidth. The transmitted signal from each terminal thus acts as interference to those from other terminals in the system. To minimize interference and increase system capacity, the transmit power of each terminal is adjusted such that the required received signal quality is maintained while minimizing interference to other terminals. On the forward link, the transmit power for the signal sent to each terminal is also adjusted so that more terminals may be served given a fixed amount of total transmit power. For some CDMA systems, a terminal may be required to be able to adjust its output power over a range of 85 dB or more.
When the output signal is at a high power level, the amount of LO leakage relative to the output signal is normally small, even for the direct conversion transmitter. However, when the output signal level is reduced, the LO leakage becomes more significant. In fact, the quality of the output signal is degraded as the amount of LO leakage approaches the desired signal level. For a direct conversion transmitter required to provide a wide range of output power, LO leakage needs to be properly addressed to ensure that the degradation due to LO leakage is acceptable even at the minimum output power level.
There is therefore a need in the art for techniques to mitigate LO leakage on an output signal when performing direct conversion.